In contrast with conventional batteries which comprise a flammable, organic liquid electrolyte, solid-state batteries comprise a solid-state electrolyte.
Said solid-state batteries are far lower risk in the event of freezing or heating, and can therefore generally be used in a significantly larger temperature range.
Due to the safety advantages thereof, in particular with respect to possible applications in larger assemblies, such as in battery-operated vehicles or as storage units for renewable energy sources, interest in said solid-state batteries has been increasing in recent years.
Although the developments are not comparable with those of lithium batteries, all sodium solid-state electrolyte batteries could nonetheless be a realistic alternative, since, in contrast with lithium, sodium, as a raw material, is available in larger quantities and is significantly cheaper.
This alone is of great interest for storing renewable energy, such as solar or wind energy, since a huge requirement therefor is predicted.
The ß/ß″-aluminates, which are already commercially available as well developed conductors of sodium ions, are among the possible options for sodium ion-conducting solid-state electrolytes.
However, the 2-dimensional ionic conductivity, and handling difficulties, result in some problems in preparation and practical application.
Partially substituted Na3Zr2(SiO4)2(PO4) ceramics are known as other options for sodium ion-conducting solid-state electrolytes that do not have the above-mentioned disadvantages, which ceramics are suitable for use as sodium ion-conducting solid-state electrolytes in solid-state sodium batteries.
Na1+xZr2(SiO4)x(PO4)3-x was discovered 40 years ago.
All modifications crystallize into a hexagonal rhombohedral structure (space group R3c), apart from in the range of 1.8≤x≤2.2, where, at room temperature, a disruption was identified in the monolithic space group C2/c.
On account of the high sodium ion conductivity thereof, these configurations are also known as NASICON (sodium (Na) Super ionic CONductor).
These systems typically reach their highest ionic conductivity when x=2 to 2.5.
Compounds having a NASICON structure are generally not electronically conductive.
In the Na3Zr2(SiO4)2(PO4) structure, partial substitution of the Zr4+ cation by a trivalent metal cation M3+, such as Al3+, Sc3+ or Y3+, results in a deficit of positive charge which is compensated by adding further Na+ ions and, overall, often leads to a higher conductivity.
One of the main problems of the materials based on Na3Zr2(SiO4)2(PO4) is the fact that the conductivity thereof is not sufficiently high compared with that of ß/ß″-aluminates.
While monocrystalline ß/ß″-aluminates have a conductivity of over 1·10−2 S/cm at room temperature, the conductivity of materials based on Na3Zr2(SiO4)2(PO4) is usually in the range of from 1·10−4 to 1·10−3 S/cm at room temperature.
However, direct use of monocrystalline ß/ß″-aluminates appears unlikely.
However, at room temperature the conductivities of polycrystalline ß/ß″-aluminates are in the range of from 1·10−3 to 2·10−3 S/cm and thus still above those of materials based on Na3Zr2(SiO4)2(PO4).
Although, in the materials based on Na3Zr2(SiO4)2(PO4), sodium ions are advantageously transported in all three spatial directions, in contrast with the 2-dimensional conductivity of the ß/ß″-aluminates, and, at approximately 1250° C., the process temperatures of materials based on Na3Zr2(SiO4)2(PO4) are much lower than those of ß/ß″-aluminates, the large difference in the conductivities has, up to now, prevented the commercialization of the materials based on Na3Zr2(SiO4)2(PO4).
WO 2014/052439 A1 discloses a very high conductivity of 1.9·10−3 S/cm at room temperature (25° C.) for Na4AlZr(SiO4)2(PO4).
US 2010/0297537 A1 discloses an even higher conductivity of 3·10−3 S/cm at 20° C. for a substituted Na1+xZr2(SiO4)x(PO4)3−x.
However, no further information regarding the composition is described here.
However, these last-mentioned conductivities are in the range of ß/ß″-aluminates and therefore again show the potential of materials based on Na3Zr2(SiO4)2(PO4).
Up to now, materials based on Na3Zr2(SiO4)2(PO4) have been prepared by means of conventional solid-state reactions.
In this case, corresponding starting powders having a particle size of greater than 1 μm are generally used for mixing and grinding.
The powder obtained following the solid-state reaction typically has a relatively large grain size, for example in the range of from 1 to 10 μm, and, disadvantageously, has some inhomogeneities and impurities.
WO 2014/052439 A1, for example, discloses a solid-state electrolyte composite comprising Na3+xMxZr2−xSi2PO12 where A=Al3+, Fe3+, Sb3+, Yb3+, Dy3+ or Er3+ and 0.01≤x≤3, which composite is characterized by the steps of a) crushing Na2CO3, SiO2, NH4H2PO4, a source of zirconium and a doping agent in a ball mill in order to prepare a ground powder, b) calcining the ground powder in order to prepare a calcined powder, and c) sintering the calcined powder in order to prepare a solid-state electrolyte.
US 2014/0197351 A1 describes a lithium-ion-conducting ceramic material, in which the powdery precursor material is first calcined, then ground and then sintered.
US 2015/0099188 A1 discloses a method for preparing a thin film, comprising a lithium-ion-conducting garnet material, in which a reaction mixture of garnet precursors and optionally a lithium source is applied to a substrate as a mixture or a slip and subsequently sintered, the garnet precursors reacting to form a thin, lithium-enriched film.
The alternative preparation approach by means of sol-gel synthesis, which approach is also known, occurs at the molecular level or on a nanometer basis, and accordingly consistently results in very homogenous materials.
However, this type of preparation generally requires complex, and therefore usually expensive, starting materials, as well as organic solvents and heating apparatuses.
These circumstances mean that, overall, this alternative preparation method is an expensive and time-consuming method which is usually profitable only for small ranges of application.